The present invention relates to processes using ozone to treat streams containing sulfur compounds.
Part of the air pollution and most of the odor problems that occur in kraft pulp mills are caused by xe2x80x9ctotal reduced sulfurxe2x80x9d (TRS) compounds, which are formed during pulping by the reaction of hydrosulfide/sulfide ions with wood components. TRS compounds include one or more of hydrogen sulfide, methyl mercaptan, dimethyl sulfide and dimethyldisulfide. These compounds have a strong unpleasant odor at very low concentrations and are toxic at somewhat higher concentrations. They are released from the digesters, where wood is converted to pulp, and from the evaporators of the spent pulping liquor together with vapors and may also be found in the spent liquor itself. The vapors are condensed and the resulting condensates are aqueous waste streams that contain variable amounts of TRS. These condensates are aqueous streams which are also referred to as xe2x80x9ccontaminated condensatexe2x80x9d or xe2x80x9cfoul condensatexe2x80x9d depending on the level of their odor, their TRS concentration and/or their content of spent pulping liquor.
Due to their relatively high temperature, condensates are frequently used in the pulp mill e.g. for pulp washing and/or washing of lime applied in the recovery process of the pulping chemicals. Normally the condensates are treated to reduce their TRS content prior to their use. Condensate that is not utilized in the mill is collected together with other liquid discharges and is then treated biologically before release into a river or lake. TRS can escape from the condensates during re-use or during aeration in the biological treatment and can thus cause an odor problem unless its concentration has been reduced to a very low level.
The so called xe2x80x9cCluster Rulesxe2x80x9d promulgated in the United States by the Environmental Protection Agency (EPA) in 1998 limit the emission of TRS from pulp mills. For some mills, especially those with older equipment, meeting the Cluster Rule limits may be very challenging.
The TRS emission problem can be reduced by stripping the condensates with air or steam. The stripped gas is then incinerated to convert the sulfur of the TRS compounds to sulfur dioxide, for example in the mill""s lime kiln, chemical recovery boiler or a stand-alone incinerator. In the latter case, the sulfur dioxide containing gas may be released into the atmosphere or may be scrubbed with a chemical solution that would then have to be disposed of. However, the stripper off-gas is a potential concentrated source of odor, and any mishap in handling this stream can create a very severe odor incident. Furthermore, the stripping efficiency may vary considerably depending on steam availability and pressure and the stripped condensate still contains some residual TRS, if the operating conditions are not optimized. However, under optimized conditions, steam stripping can remove almost 100% of the TRS. In contrast, air stripping is generally less efficient. In older mills, retrofitting of stripping equipment can be very expensive.
Biological treatment of effluents, both anaerobic and aerobic, is being developed to remove TRS and other organic compounds contained in the condensate. Anaerobic treatment requires considerable equipment, oxygen must be completely excluded and stringent process control is usually required due to the sensitive nature of aerobic microorganisms. Aerobic treatment requires aeration or addition of oxygen to the condensate. This mode of treatment can strip unoxidized TRS compounds into the off-gas unless special precautions are taken. Some bio-sludge has to be removed from both types of biological treatment.
Other methods proposed for TRS removal from condensates include:
activated carbon adsorption;
chemical oxidation with chlorine or oxygen;
chemical precipitation.
These three methods have not found application in the kraft pulp industry. Activated carbon adsorption and chemical precipitation generate a solid waste that needs to be further treated or perhaps disposed of in a special land-fill, while oxidation with chlorine is no longer environmentally acceptable. Oxidation with oxygen alone is of doubtful value, as shown herein.
One aspect of the present invention is a process for treating an aqueous stream containing one or more reduced sulfur compounds, comprising:
(a) reacting an aqueous feed stream containing TRS contaminant selected from the group consisting of hydrogen sulfide, methyl mercaptan, dimethyl sulfide, dimethyl disulfide, and mixtures thereof, with ozone to produce a first liquid product stream and a first gaseous product stream, wherein the total amount of said one or more contaminants in said first liquid product stream is less than 1% of the amount thereof in said feed stream, and wherein said first gaseous product stream contains ozone and one or more of said contaminants; and
(b) treating said first gaseous product stream with an alkaline aqueous stream to remove essentially all of said one or more contaminants from said first gaseous product stream.
Preferably, a second gaseous stream produced in step (b) and containing ozone is reacted with an aqueous stream containing one or more of said contaminants, to consume all the ozone in said stream.
The present invention is applicable to all types of condensates produced in a pulp mill as long as they contain reduced forms of sulfur. These include (1) highly contaminated streams obtained from wood pulping and black liquor evaporation systems. These condensates come more specifically from the flash tanks of continuous digesters, blow tank relief valves of batch digesters, turpentine recovery systems, later stages of the evaporation system (known in the industry as effects 2 and 3) and the concentrator; (2) secondary condensates coming from the earlier stages of the evaporation system (known in the industry as effects 4 and 5); (3) condensates previously purified via air stripping; and (4) condensates previously purified via steam stripping. The concentration of TRS (total reduced sulfur) present in these various condensate streams may vary from 1 mg/L to 1500 mg/L and are treatable by the process of this invention. The stream being treated may also contain one or more of carbonyl sulfide, methanol, ethanol, acetone, furfural, and butanone.
The process is also applicable to contaminated condensates from other industrial operations as long they contain TRS concentrations within the range above specified.
The present invention achieves the destruction of TRS contained in condensates with ozone followed by scrubbing of the off-gas from the ozone treatment with an aqueous alkaline stream which can be a waste stream from the pulp mill. A large portion of the TRS can be oxidized with ozone, for example to sulfate, while a smaller amount of said contaminants is removed by being stripped into the oxygen rich off-gas. With a sufficiently high ozone dosage, TRS is removed so completely from the condensate that the amount of said contaminants remaining in the liquid stream produced in this step is less than 1%, preferably less than 0.5%, and more preferably less than 0.1% of the amount originally present in the stream fed to this step. At the same time, the color of condensates that are contaminated with small quantities of spent pulping liquor (so-called black liquor) decreases significantly; the pH of these contaminated condensates is reduced, from alkaline values typically about 9 or higher to 7.5 or less (preferably 7 or less) and their dissolved oxygen content rises significantly. Due to this quality improvement, the condensate may then be used in the mill without causing TRS emission problems.
The ozone used for condensate decontamination may be produced from an on-site generation system at concentrations ranging from 2-20% in the gas stream. However, the off-gas from an ozone bleaching system can also be directly applied in the decontamination of condensates. In addition, combinations of ozone with other oxidants and/or UV light could also be used. The impurities contained in this off-gas have little impact on the reaction of the leftover ozone with the sulfur compounds. The dosage of ozone applied will depend on the concentration of TRS present in the condensate. In general, the ozone doses will vary in the range from 10 to 1500 mg/L of condensate. The amount of ozone fed relative to the amount of said contaminants fed should be at least stoichiometric. Preferably, the ozone fed to the stream containing the contaminants should be in the ratio of at least 2:1, and more preferably at least 4:1, expressed as (moles of ozone):(moles of TRS contaminants as S).
The type of reactor that can be used to contact the ozone with the condensates may vary substantially. Column reactors are suitable but tubular and high-shear mixers can also be used. These reactors are well known to those skilled in the art and are available in the market from various equipment suppliers.
The operating conditions to react ozone with the condensates may vary substantially depending upon the type of condensates and pulp mill. Ideally, the reaction should take place at the temperature and pH of the condensate stream, to avoid the costs incurred with pH and temperature control of a large volume of condensate. In general, the range of pH at which the reaction of ozone with condensate should occur is in the range of 8-12 and the temperature is in the range of 40-90xc2x0 C. The contact time between ozone and condensate will vary from 1-120 min depending upon the concentration of TRS present in the condensate.
The off-gas stream produced in this ozone treatment step contains 1-15 mg/L of the contaminants hydrogen sulfide, methyl mercaptan, dimethyl sulfide, and dimethyl disulfide. This stream is then fed to a second step where it is scrubbed with a dilute alkaline solution, to absorb the stripped TRS and any sulfur dioxide or trioxide formed by oxidation of TRS contained in the condensate. Examples of dilute alkaline solutions found suitable for scrubbing are alkaline bleach plant effluents from an extraction stage of the pulp mill or from a peroxide bleaching stage of the pulp mill. That is, peroxide compounds are an optional but preferred component of this stream. The residual peroxide from bleaching may further oxidize any stripped TRS or SO2, thereby eliminating the residual peroxide, which can be environmentally harmful. The bleach plant effluent is then treated biologically in the normal manner together with other pulp mill effluents.
The alkaline solution that can be used to scrub any stripped TRS from the gaseous stream produced in the ozone/condensate reactor may include oxidized white liquor, sodium hydroxide or preferably alkaline bleach plant filtrate. Oxidized white liquor exists in pulp mills and is used in oxygen delignification processes. Sodium hydroxide solutions are used in alkaline extraction stages and alkaline bleach plant filtrates originate from hypochlorite beaching, conventional alkaline extractions, oxidative extractions with oxygen, oxidative extractions with peroxide, oxidative extractions with oxygen and peroxide, conventional peroxide stages and pressurized peroxide stages. The presence of residual peroxide and hypochlorite in these filtrates is favorable to eliminate in this stage any TRS contaminants stripped out from the ozone/condensate reactor in the first stage.
The concentration of alkali in these various alkaline solutions may vary substantially among the various sources. However, it is required that the pH of these solutions be in the range of 9-14 in order to scrub any fugitive TRS. The scrubbing of the off-gas stripped out from the ozone/condensate reactor with alkaline solution will take place at the temperature and pH of the alkaline stream used for such purpose. The pH of these streams should be in the range of 9-14 and the temperature in the range of 40-90xc2x0 C. The alkali scrubbing of the stripped off-gas generally takes place in 1-120 min reaction time.
Alkaline solutions of pH 9-14 can also be used to neutralize the treated condensates from the first stage prior to piping them to the effluent treatment station.
The off-gas stream from this second step often contains residual ozone. It is preferred to contact this stream with condensate containing one or more of said reduced sulfur contaminants, either by carrying out contact in a third step in a suitable column or by recycling the off-gas stream to the condensate in the first step of the invention.
The technical advantages of the present invention include a very high oxidation potential, very rapid reaction, direct action on the TRS compounds, oxygen as end reaction and decomposition product, and complete absence of solid waste formation such as sludge. In addition, except for ozone, no fresh chemicals are needed.
The present invention presents special advantages when employed in connection to a pulp mill, because existing process streams generated by the conventional operations of the pulp mill can be employed without requiring any pretreatment thereof.
The simplicity of the process is apparent from the examples listed below, which are all based on well controlled laboratory experiments and which were conducted with condensate samples from three different pulp mills, which are referred to herein as mills A, B and C.
In the laboratory, one or two liter condensate samples were treated with ozone in a vertical glass reactor (No. 1) equipped with an inlet pipe and sparger close to the bottom of the reactor, a magnetic mixer at the bottom end and an outlet pipe at the top end, at the temperature measured during sampling. The flow rate of the ozone was well controlled at the desired levels, the foam formed during the ozone treatment was collected in one or two 1-liter traps which were of similar design as the reactor, and the collapsed foam was mixed with the treated condensate immediately after the reaction. The first set of preliminary experiments was carried out with only one reactor (reactor 1) to evaluate ozone potential to reduce TRS. In the second set of experiments, the off-gas from the traps was scrubbed in two consecutive steps with water (reactor 2) and a dilute sodium hydroxide solution (reactor 3) to absorb TRS and/or sulfur trioxide that may be contained in the off-gas. These reactors were followed by another trap to collect any carry-over from the alkaline scrubber. In later experiments, the water scrubber (reactor 2) was eliminated and the fresh sodium hydroxide (reactor 3) was replaced with alkaline bleach effluent preheated to the temperature measured during sampling in the mill. Finally, the scrubbed gas was fed to a vertical plastic column filled with an aqueous solution of potassium iodide (KI) to react completely with the residual ozone or with iodine (I2) at pH 5 to react with residual TRS, for analytical purposes. In all cases, sulfide contaminants denoted xe2x80x9cTRSxe2x80x9d contained one or more of hydrogen sulfide, methyl mercaptan, dimethyl sulfide, and/or dimethyl disulfide.